Eliminate if‑else in Spring Boot with the Service Locator Pattern

1. Introduction

In many projects you need to invoke different implementations based on the input type, such as using separate parsers for XML, JSON, and CSV files. Traditional if‑else statements quickly become hard to maintain.

This article shows how to solve this problem in Spring Boot 3.4.2 using the Service Locator pattern.

2. Practical Example

2.1 Define Enum

public enum ContentType {
  JSON(TypeConstants.JSON_PARSER),
  XML(TypeConstants.XML_PARSER),
  CSV(TypeConstants.CSV_PARSER);

  private final String parserName;
  ContentType(String parserName) { this.parserName = parserName; }
  @Override
  public String toString() { return this.parserName; }

  public interface TypeConstants {
    String CSV_PARSER = "csvParser";
    String JSON_PARSER = "jsonParser";
    String XML_PARSER = "xmlParser";
  }
}

2.2 Define Parser Interface

public interface Parser {
  Map<String, Object> parse(Reader r);
}

2.3 Implement Parsers

@Component(TypeConstants.CSV_PARSER)
public class CSVParser implements Parser {
  @Override
  public Map<String, Object> parse(Reader r) {
    return Map.of("csv", "csv文件解析成功");
  }
}

@Component(TypeConstants.JSON_PARSER)
public class JSONParser implements Parser {
  @Override
  public Map<String, Object> parse(Reader r) {
    return Map.of("json", "json文件解析成功");
  }
}

@Component(TypeConstants.XML_PARSER)
public class XMLParser implements Parser {
  @Override
  public Map<String, Object> parse(Reader r) {
    return Map.of("xml", "xml文件解析成功");
  }
}

2.4 Define Service Locator Interface

public interface ParserFactory {
  Parser getParser(ContentType contentType);
}

2.5 Configure ServiceLocatorFactoryBean

@Configuration
public class ParserConfig {
  @Bean("parserFactory")
  ServiceLocatorFactoryBean serviceLocatorFactoryBean() {
    ServiceLocatorFactoryBean factoryBean = new ServiceLocatorFactoryBean();
    factoryBean.setServiceLocatorInterface(ParserFactory.class);
    return factoryBean;
  }
}

Spring generates a proxy for

ParserFactory

; the underlying

InvocationHandler

uses the first method argument to resolve the appropriate bean name.

2.6 Use Parser Service

@Service
public class ParserService {
  private final ParserFactory parserFactory;
  public ParserService(ParserFactory parserFactory) { this.parserFactory = parserFactory; }
  public Map<String, Object> getData(ContentType contentType) {
    Parser parser = parserFactory.getParser(contentType);
    InputStreamReader reader = null; // replace with actual source
    return parser.parse(reader);
  }
}

2.7 Test Runner

@Component
public class ParserRunner implements CommandLineRunner {
  private final ParserService parserService;
  public ParserRunner(ParserService parserService) { this.parserService = parserService; }
  @Override
  public void run(String... args) throws Exception {
    Map<String, Object> data = parserService.getData(ContentType.CSV);
    System.err.println(data);
    data = parserService.getData(ContentType.JSON);
    System.err.println(data);
    data = parserService.getData(ContentType.XML);
    System.err.println(data);
  }
}

Running the application prints the parsing results for CSV, JSON and XML.

Conclusion

The Service Locator pattern removes the client’s direct dependency on concrete implementations. As Martin Fowler puts it, “the basic idea is to have an object that knows how to obtain all services an application may need.”